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Showing papers in "Surface & Coatings Technology in 2002"


Journal ArticleDOI
TL;DR: In this paper, an assessment of the mechanical behavior of hard and superhard nanocomposite coatings from loading/unloading curves measured by a computer-controlled Fischerscope H 100 microhardness tester and a maximum depth d max of the diamond indenter impression into the coating at a given load L.
Abstract: The paper is devoted to an assessment of the mechanical behavior of hard and superhard nanocomposite coatings from loading/unloading curves measured by a computer-controlled Fischerscope H 100 microhardness tester and a maximum depth d max of the diamond indenter impression into the coating at a given load L . It is shown that: (1) the area between the loading/unloading curve and the value of d max decreases with increasing (i) hardness H , (ii) effective Young's modulus E * = E /(1−ν 2 ) and (iii) universal hardness HU, where E and ν are the Young's modulus and the Poisson ratio, respectively; and (2) there is no simple relation between the mechanical response of the coating and H or E * alone; however, this response is strongly dependent on the ratio H / E * . The last fact gives a possibility of tailoring the mechanical properties of a coating for a given application, e.g. to prepare coatings with high hardness H , high resistance to plastic deformation (∼ H 3 / E *2 ), high elastic recovery W e , but with low E * and high d max . Special attention is also given to the analysis of problems in accurately measuring the hardness of superhard (≥60 GPa) coatings. It is shown that a high elastic recovery W e ≥80% of superhard films with H ≥60 GPa (1) strongly decreases the gradient d H /d L and (2) shifts the region L , where H ( L )≈constant and the hardness H is correctly measured, to higher values of L . This means that the lowest load L used in the hardness measurement must be higher than L used in measurements of coatings with H H measured from being significantly higher than the real hardness of the coating.

576 citations


Journal ArticleDOI
TL;DR: In this paper, the advantages and disadvantages of various thermal barrier coating (TBC) systems are discussed, with the aim of custom designing a TBC system to be both strain tolerant and have a low thermal conductivity.
Abstract: This paper reviews the advantages and disadvantages of various thermal barrier coating (TBC) systems, with the aim of custom designing a TBC system to be both strain tolerant and have a low thermal conductivity. Methods of heat transfer within zirconia based ceramics are discussed, including the influence of coating microstructure and ceramic composition. It is shown the addition of dopant atoms (colouring) is effective in reducing ‘phonon’ transport and that layered microstructures are effective in reducing ‘photon’ transport. Advanced processing, using EB-PVD coating methods has allowed both coloured and layered ceramic coatings to be produced. Measured thermal conductivities of 1.0 W mK −1 have been achieved using these methods, much lower than current commercial EB-PVD coatings at 1.5–1.9 W mK −1 .

412 citations


Journal ArticleDOI
TL;DR: In this article, the authors used X-Ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate the coating microstructure, and the coating/substrate interface.
Abstract: Alumina coatings were deposited on Al alloy substrates using an electrolytic plasma technique, based on a dielectric barrier discharge created during anodic oxidation in an aqueous electrolyte. The substrate material (BS Al 6082) was biased anodically with an unbalanced AC high voltage. During processing, a plasma current density of 100 mA/cm2 was used, at which a coating deposition rate of 1.67 μm/min was achieved. Coating abrasive wear and corrosion properties were assessed by conducting dry and wet rubber wheel abrasive tests and potentiodynamic polarization experiments, respectively. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the coating microstructure, and the coating/substrate interface. The property test results show that the coatings possess excellent abrasive wear and corrosion resistance. XRD analyses indicate that the coatings consist of α- and γ-Al2O3. An amorphous+nanocrystalline inner layer (1.5-μm thick) and a nanocrystalline (50–60 nm) intermediate layer in the coating were observed by TEM. The higher resistance to wear and corrosion can in part be attributed to the presence of these interlayers.

395 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive model for kinetic spray coating formation is proposed based on observations reported in this paper, and four velocity-dependent stages of coating formation are proposed based upon observations reported here, which are consistent with a metallic form for tensile tests on Al coatings removed from the substrate.
Abstract: Coatings have been produced by entraining relatively large diameter metal powders in a supersonic airflow. For the first time, most of the particles in the powders have diameters >50 μm. Substantial plastic deformation is involved in the conversion of the particle's kinetic energy into heat and strain energy in this kinetic spray process. As suggested by simple estimates and confirmed by coating grain structures, the particles are not melted or thermally softened in this coating process. These coatings have a relatively low oxide content, low thermal stress, high adhesion, low porosity and hardness somewhat higher than those of corresponding bulk materials. Threshold or critical velocities for coating formation are discussed. Critical velocities for the relatively large particles were observed to be substantially less than have been reported earlier for smaller diameter (<50 μm) particles. Coating particle rotation and deformation due to particle impact resulted in a corresponding decrease in porosity. Bond formation, particle deformation and grain deformation were found to be highly anisotropic, depending on the direction of the incident particle velocity. At higher incident velocities, increasing metallic bond formation between particles was observed. This is consistent with a metallic form for stress/strain curves obtained via tensile tests on Al coatings removed from the substrate. The coating elastic modulus was found to be less than half that of bulk Al. Measured ultimate tensile strengths and yield points of Al coatings were comparable to those of bulk Al. This may be due to work hardening resulting from the plastic deformation necessary for coating formation. These tensile test results are consistent with coating cohesive strengths as measured by stud pull tests. Higher powder feed rates produced coatings with higher failure loads in three point bending, higher coating cohesion and lower coating strength anisotropy, presumably due to a peening effect. Four velocity-dependent stages of coating formation are proposed based on observations reported here. Coating properties arise from a competition between these stages. Parallels with models of dynamic (explosive) powder compaction are made. This is the first comprehensive model for kinetic spray coating formation.

331 citations


Journal ArticleDOI
TL;DR: In this article, the effect of ion beam-induced compositional and structural changes and formation of superstoichiometric nitride compounds was analyzed in detail by X-ray photoelectron spectroscopy (XPS).
Abstract: Nitride coatings have been used in numerous applications to increase the hardness and improve the wear and corrosion resistance of structural materials, as well as in various high-tech areas, where their functional rather than mechanical properties are of prime importance. Performance of these coatings is equally dependent on their chemical composition and long-range crystalline structure, as well as on the nature and amount of impurities and intergranular interactions. Significant improvement in the mechanical properties has recently been achieved with multi-component superlattice and nanocomposite nitride coatings. In the case of such multi-component systems, not only is close control of the elemental composition (stoichiometry) necessary to optimize the properties of the coatings, but the influence of chemical bond formation between the components is also of prime importance. Special care needs to be taken when non-equilibrium preparation conditions, activation of CVD and PVD by plasmas or energetic particle beams are applied, occasionally leading to unpredicted deviations, both in composition and structure. As is highlighted in this paper, nitride coatings or nitrided surfaces can be analyzed in detail by X-ray photoelectron spectroscopy (XPS) due to its excellent element selectivity, quantitative character and high surface sensitivity. More importantly, XPS reflects the atomic scale chemical interactions, i.e. the bonds between neighboring atoms, and thus it also provides reliable structural characteristics for amorphous or nano-crystalline coatings of complex composition, for which application of diffraction techniques is not straightforward. A number of examples of the application of XPS are given for various types of nitride coatings, including interstitial compounds, such as TiN, CrNx, etc., as well as compounds with predominantly covalent bonding, such as AlN, GaN, Si3N4 and CNx. Special emphasis is placed on ion beam-induced compositional and structural changes and to the formation of ‘superstoichiometric’ TiN1+x, ZrN1+x compounds.

292 citations


Journal ArticleDOI
TL;DR: Inorganic Fullerene-like (IF)-MoS2 nanoparticles were tested under boundary lubrication and ultra-high vacuum (UHV) and were found to give an ultra-low friction coefficient in both cases compared to hexagonal MoS2 material as mentioned in this paper.
Abstract: Inorganic Fullerene-like (IF)-MoS2 nanoparticles were tested under boundary lubrication and ultra-high vacuum (UHV) and were found to give an ultra-low friction coefficient in both cases compared to hexagonal (h)-MoS2 material. Previous works made by Rapoport et al. with IF-WS2 revealed that the benefit effect of the inorganic fullerene-like materials decreases at high loads and sliding velocities. Nevertheless, under the conditions used in our experiments using high contact pressure (maximum pressure above 1.1 GPa in oil and 400 MPa in high vacuum) and slow sliding velocities (1.7 mm/s in oil test and 1 mm/s in high vacuum), friction always decreases and stabilizes at about 0.04 for 800 cycles in both cases. Therefore, IF-MoS2 material appears to be a good candidate for use in various environments in regard to other MoS2 crystal structures. Wear mechanisms were investigated using both High Resolution TEM and surface analyses (XPS) on the wear tracks. Wear particles collected from the flat wear scar show several morphologies, suggesting at least two lubricating mechanisms. As spherical particles are found in the wear debris, rolling may be a possible event. However, flattened and unwrapped IF-MoS2 particles are often observed after friction. In this case, low friction is thought to be due either to sliding between IF-MoS2 external flattened planes or to slip between individual unwrapped MoS2 sheets.

266 citations


Journal ArticleDOI
Yongjun Zhang1, Chuanwei Yan1, Fuhui Wang1, Hanyi Lou1, Chunan Cao1 
TL;DR: In this paper, a new anodizing process based on an environmentally friendly electrolyte solution that contains none of chromate, phosphate or fluoride but can enhance the corrosion protection of magnesium alloy significantly, is investigated.
Abstract: A new anodizing process, based on an environmentally friendly electrolyte solution that contains none of chromate, phosphate or fluoride but can enhance the corrosion protection of magnesium alloy significantly, is investigated. Anodizing behaviors of magnesium and its alloys are influenced by many factors, including the constituents and concentrations of electrolyte solution, the nature of basis metal as well as applied current density or voltage, treatment time and solution temperature, etc. Therein, the electrolyte solution plays a determinant role. Applied constant current can ensure that anodic films grow at an almost uniform rate. Higher current density and lower solution temperature benefit the film growth. Higher voltage indicates thicker film when constant current is provided. However, the voltage does not always increase linearly with increasing film thickness even under the condition of constant current density. The new anodic film, having a number of pores that are relatively uniform in size as well as distribution on a microstructure scale and do not tranverse the entire film, forms a relatively intact and compact barrier layer. By comparison with the films produced by two available classic processes, the new film can provide more effective corrosion protection to substrate.

245 citations


Journal ArticleDOI
TL;DR: In this article, the optimization of powder preparation for thermal spraying by spray drying and the development of parameters for atmospheric plasma spraying were described in order to produce homogeneous crystalline coatings with controlled micro-porosity and residual stresses.
Abstract: Lanthanum hexaaluminate (LHA) with a magnetoplumbite structure is a promising competitor to yttria partially stabilized zirconia (Y-PSZ) as a thermal barrier coating (TBC), since most zirconia coatings age significantly, including undesired densification at temperatures exceeding 1100 °C. The microstructure of calcined lanthanum hexaaluminate powders and thermally sprayed coatings show a platelet structure. The magnetoplumbite structure is characterized by the highly charged La 3+ cation located in an oxygen position in the hexagonal close-packed structure of oxygen ions. Ion diffusion is strongly suppressed vertical to the crystallographic c -axis, thus hindering sintering densification. In contrast to the oxygen ion conducting zirconia, lanthanum hexaaluminate permits operating temperatures above 1300 °C because of its thermal stability and electrically insulating properties. This study describes the optimization of powder preparation for thermal spraying by spray drying and the development of parameters for atmospheric plasma spraying (APS) in order to produce homogeneous crystalline coatings with controlled micro-porosity and residual stresses. The phases were characterized by X-ray diffraction (XRD).

205 citations


Journal ArticleDOI
TL;DR: In this article, the effect of total pressure and the oxygen partial pressure on the deposition rate, the phase composition, the crystallinity, the surface morphology and the resulting photocatalytic properties was investigated.
Abstract: Crystalline titanium dioxide, TiO2, photocatalytic films were deposited by reactive r.f. magnetron sputtering on glass substrates without additional external heating. A pure metallic titanium target was sputtered in a mixture of argon and oxygen. The effect of the total pressure and the oxygen partial pressure on the deposition rate, the phase composition, the crystallinity, the surface morphology and the resulting photocatalytic properties was investigated. The films were characterized by X-ray diffraction, scanning electron microscopy and scanning probe microscopy. The photocatalytic activity was evaluated by the measurement of the decomposition of methylene blue under UV irradiation. The results showed that the crystalline anatase, anatase/rutile or rutile films can be successfully deposited on unheated substrate and their formation is dependent on the total pressure and the oxygen partial pressure. A schematic phase diagram was constructed. The surface morphology is strongly influenced by the total pressure and the anatase TiO2 films with a more open surface, a higher surface roughness and a larger surface area are formed at higher total pressures. The anatase films with such surface morphology deposited in the reactive sputtering mode exhibit the best photocatalytic activity.

205 citations


Journal ArticleDOI
TL;DR: In this paper, the evolution of the electron, energy distribution and the plasma parameters in a high-density plasma in a pulsed magnetron discharge is demonstrated. And the high density plasma is created by applying applying...
Abstract: We demonstrate the evolution of the electron, energy distribution and the plasma parameters in a high-density plasma in a pulsed magnetron discharge. The high-density plasma is created by applying ...

203 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used X-ray diffraction (XRD) to determine the preferred orientations of TiN films with different film thickness and found that the dominant preferred orientation of the TiN coatings for the deposition conditions was (111), especially for the films thicker than 1 μm.
Abstract: Titanium nitride (TiN) film was deposited on 304 stainless steel using a hollow cathode discharge ion-plating (HCD-IP) technique. Film thickness and N/Ti ratio were controlled. The depth profile of the composition was determined by secondary ion spectroscopy (SIMS). The results showed that the compositions of the films were uniform for the films deposited at the same deposition conditions. The purpose of this study is to investigate the variation of structure and mechanical properties of the TiN films with different film thickness. The preferred orientations of TiN films were determined using X-ray diffraction (XRD). The dominant preferred orientation of the TiN coatings for the deposition conditions was (111), especially for the films thicker than 1 μm. The residual stress of the TiN films was also measured by XRD using sin2 Ψ method. The residual stress was ranging from −5.93 to −2.70 GPa, varying with film thickness. Hardness of the films was measured by nanoindentation. The hardness values were ranging from 14.9 to 33.6 GPa, increasing with the film thickness. The ultimate interfacial shear stress between TiN/304SS was determined by in-situ strip tension of the TiN-coated specimens in a scanning electron microscope (SEM) chamber. The limiting thickness for the effective measurement of interfacial shear strength by the in-situ strip tension method is close to 0.5 μm. N/Ti ratios of the thin films were all at 0.8 measured using both X-ray photoelectron spectrometer (XPS) and Rutherford backscattering spectrometer (RBS). The packing factors of TiN films, calculated from the results of RBS, were 0.62–0.99, increasing with film thickness and leveling off at a thickness above 1.2 μm.

Journal ArticleDOI
TL;DR: Smart overlay coatings as mentioned in this paper are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions, such as high temperature oxidation and type I hot corrosion.
Abstract: Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions. The SMARTCOAT design consists of a MCrAlY base, enriched first in chromium, then aluminium to provide a chemically graded structure. At elevated temperatures, above 900°C (1650°F), the coating oxidises to form a protective alumina scale. However, at lower temperatures this alumina scale does not reform rapidly enough to confer protection under type II hot corrosion conditions. The coating is therefore designed with an intermediate chromium-rich interlayer, which permits the rapid formation of chromia healing areas of type II corrosion damage. Laboratory and burner rig tests have been carried out on a series of developmental smart overlay coatings. These have shown that the development of an intermediate chromium-rich phase provides protection under low temperature hot corrosion conditions, while the aluminium-rich surface layer provides resistance to high temperature oxidation and type I hot corrosion. Thus, the single application of SMARTCOAT permits operation over a broad range of industrial and marine turbine conditions.

Journal ArticleDOI
TL;DR: In this article, a systematic study on the relationship between the structure and abrasive wear resistance of autocatalytic nickel-phosphorus coatings (particle-free and SiC composite) with different phosphorus contents and under different thermal treatments (i.e. 300, 400 and 500°C) has been performed.
Abstract: A systematic study on the relationships between the structure and abrasive wear resistance of autocatalytic nickel–phosphorus coatings (particle-free and SiC composite) with different phosphorus contents (i.e. 2.5–10.2 wt.% P) and under different thermal treatments (i.e. 300, 400 and 500°C) has been performed. The phase structure, composition and properties of the coatings could be controlled by changing the phosphorus content of the nickel–phosphorus matrix and by performing thermal treatments. The improvement in abrasive wear behaviour of the nanocrystalline (i.e. ≤6.0 wt.% P) coatings with heat treatment temperature up to 400°C was related to (i) the formation of a metastable equilibrium phase and (ii) precipitation of Ni 3 P compound. At higher thermal treatments (500°C), a change in the deformation mechanisms (Orowan mechanism) determined by the coarsening of Ni 3 P precipitates was associated with the decrease in abrasive wear resistance of the coatings. In addition, for the NiP–SiC coatings after annealing at 500°C, Ni 3 Si was formed and the adhesion between the reinforcement and the matrix was enhanced.

Journal ArticleDOI
TL;DR: Cerium-based conversion coatings were formed by a spontaneous reaction between a water-based solution containing CeCl3 and aluminum alloy 7075-T6 substrates in this article.
Abstract: Cerium-based conversion coatings were formed by a spontaneous reaction between a water-based solution containing CeCl3 and aluminum alloy 7075-T6 substrates. Coating performance was evaluated in neutral salt fog according to ASTM B117. Coating microstructure and thickness were observed by scanning electron microscopy (SEM). Coating composition and the cerium oxidation state were characterized using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) respectively. The morphology and salt fog performance of cerium conversion coatings were affected by pre-treatment of the panel prior to coating. The best pre-treatment consisted of desmutting, degreasing, and acid activation. After immersion in the coating solution for 30 s, Ce-rich deposits formed on the 7075 surface. After 5 min, coatings consisted of Ce-rich particles in a Ce-containing matrix. Immersion times of 5 min or longer produced coatings that could pass published military requirements for conversion coating performance in neutral salt fog. XPS analysis showed that the coatings contained Ce4+.

Journal ArticleDOI
TL;DR: The results of scratch tests show that the Ni-B deposits have good tribological properties under external solicitations as discussed by the authors, they are hard, wear and abrasion resistant, and also have good adhesion to the aluminium substrate.
Abstract: Electroless nickel–boron baths reduced with sodium borohydride can be stabilized with various agents such as thallium nitrate or lead tungstate with no fundamental modification of deposition rates and stability. To improve the mechanical properties of electroless nickel–boron deposits, various heat treatments are applied. At low temperatures, no fundamental changes in the deposit structure are observed, only an improvement of adhesion on aluminium substrate. The values of the Knoop microhardness obtained on these heat-treated deposits are near 600 hk 100 . At higher temperatures, structural changes take place and the nickel–boron deposits crystallize. The microhardness rises until 1050 hk 50 for heat treatments at 350 °C for 4 h. A diffusion layer between the electroless nickel deposit and the aluminium substrate appears at high heat treatment temperatures. The results of scratch tests show that the Ni–B deposits, with or without heat treatments, have good tribological properties under external solicitations. They are hard, wear and abrasion resistant, and also have good adhesion to the aluminium substrate.

Journal ArticleDOI
TL;DR: In this paper, the effect of the ratio of a cationic surfactant concentration to PTFE concentration in the plating solution on the electroless Ni-P-PTFE deposition rate and on the pTFE content in the coatings has been investigated.
Abstract: The effect of the ratio of a cationic surfactant concentration to PTFE concentration in the plating solution on the electroless Ni–P–PTFE deposition rate and on the PTFE content in the coatings has been investigated. The adhesion of the Ni–P–PTFE layer is significantly improved by gradually increasing the PTFE content from the substrate to the top surface. It has been demonstrated that these graded electroless Ni–P–PTFE coatings can reduce the formation of deposits on heat exchanger surfaces significantly.

Journal ArticleDOI
TL;DR: In this article, the optimum pack thickness required to form boride coating of adequate thickness and property in the case of a low carbon steel boronized at 940°C for 2 h.
Abstract: Boronizing, which involves diffusion of boron atoms into steel substrate to form iron borides, is a well-known diffusion coating process and numerous studies have demonstrated the outstanding tribological properties of boronized steel vis-a-vis carburized or nitrided steels However, the high cost of the boronizing process has severely limited its applications One way to bring down the cost of the boronizing process is to reduce the thickness of the boronizing mixture to be packed around the component (called pack thickness) to the minimum required level without compromising on the properties of the boride coating The present study attempts to estimate the optimum pack thickness required to form boride coating of adequate thickness and property in the case of a low carbon steel boronized at 940°C for 2 h Low carbon steel samples have been boronized with varying pack thickness in the range 2-25 mm and the resulting boride coatings have been examined for thickness, microstructure, microhardness profile and abrasion resistance An analysis of the results obtained indicated that a pack thickness of 10 mm is sufficient to obtain boride coatings of adequate thickness and optimum properties

Journal ArticleDOI
TL;DR: The phase composition of micro-arc oxidized coatings has been examined using X-ray diffraction, scanning electron microscopy, and Xray photoelectron spectroscopy as discussed by the authors.
Abstract: The coating deposition mechanism of aluminum alloys by means of micro-arc oxidation has been investigated, which may be adapted to the further application of wear resistant protection of Al alloys. The phase composition of the micro-arc oxidized coatings has been examined using X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The antiwear behavior of the coatings was evaluated with a scratch tester, reciprocating tester and Timken tester. The results show that the micro-arc oxidized coatings on Al alloys show two distinct regions, i.e. a porous overlayer region consisting predominantly of γ-Al2O3 which was produced at the higher cooling rate, and a dense internal region consisting predominantly of α-Al2O3 which was generated at the lower cooling rate. They are well bonded on the Al substrate in 40–110 μm coatings. The polished coating mainly composed of α-Al2O3 registers a lower wear of 3.00–5.00×10−6 mm3/N m in reciprocating sliding against ceramic counterpart at a speed of 0.33 m/s and a contact pressure of 2 MPa. The antiwear life of the polished coating reached 2500 m at a speed of 1.25 m/s and load of 300 N and the friction coefficient was more than 0.45 against the steel ring in a Timken tester. Thus, the micro-arc oxidation coating mainly composed of α-Al2O3 could be a promising candidate as a protective coating of Al alloy-based components in terms of wear-resistance, and further improved lubrication effects as a self-lubrication coating or using oil, grease and solid lubrication films.

Journal ArticleDOI
TL;DR: In this article, the effects of processing parameters on cleaning steel surfaces (oxides and contamination) and deposition of Zn and Zn-Al coatings were investigated, and the results show that EPP can effectively produce clean surfaces and also metal and alloy coatings at high deposition rates, and has a great potential as a new plasma surface engineering technique.
Abstract: Electrolytic plasma processing (EPP) involves electrolysis and electrical discharge phenomena and it is an emerging, environmentally friendly surface engineering technology. Electrolytic-plasma/material surface interactions during processing can be used for cleaning of metal surfaces, formation of diffusion layers and/or deposition of metal, ceramic and composite coatings. The present work was concerned with cleaning and deposition of metal coatings on steel surfaces for corrosion protection. The effects of processing parameters on (i) cleaning steel surfaces (oxides and contamination); and (ii) Zn and Zn–Al coating deposition were investigated. Surface roughness and oxygen content prior to and after cleaning were evaluated by profilometry and energy dispersive X-ray analysis (EDAX), respectively. The structure of the EPP cleaned outer surface layer as it evolves after the electrolytic–plasma interaction was studied by high resolution TEM. Morphology, microstructure, composition, adhesion and density of EPP-deposited Zn and Zn–Al coatings on cleaned surfaces were studied as a function of processing parameters. Corrosion properties of the cleaned and coated steels were evaluated by corrosion potential and potentiodynamic polarization measurements. The results show that EPP can effectively produce clean surfaces and also metal and alloy coatings at high deposition rates, and it has a great potential as a new plasma surface engineering technique.

Journal ArticleDOI
TL;DR: In this article, a series of experiments were performed to evaluate some mechanical properties of boronized AISI W4 steel, and it was found that the hardness of the boride layers ranged from 1407 to 2093 HV.
Abstract: A series of experiments was performed to evaluate some mechanical properties of boronized AISI W4 steel. Boronizing was carried out in a solid medium consisting of EKabor powders at 850, 950 and 1050°C for 2, 4, 6 and 8 h. After boronizing, FeB and Fe 2 B phases were formed on the surface of the steel substrate. A boride layer was revealed by a classical metallographic techniques and X-ray diffraction (XRD) analysis. Depending on the process temperature and boronizing time, the thickness of the coating layers ranged from 8 to 386 μm. Metallographic studies revealed that the boride layer has a lenticular morphology. The hardness of the boride layer was measured using a Vickers indenter with loads of 0.5 and 1 N. It was found that the hardness of the boride layers ranged from 1407 to 2093 HV. The fracture toughness of borided surfaces was measured via a Vickers indenter with a load of 10 N. It was observed that the fracture toughness of the boride layer ranged from 1.39 to 6.40 MPa m 1/2 . A longer boronizing time results in a greater boride layer thickness. Lengthwise cracks were formed on the samples that were borided at 1050°C for 6 and 8 h. The distribution of alloying elements from the surface to the interior was determined using energy-dispersive X-ray spectroscopy (EDS). The main aim of present study was to increase the service life of AISI W4 plain carbon tool steel.

Journal ArticleDOI
TL;DR: In this paper, three possible causes of internal delamination failures in thermal barrier coating (TBC) systems are analyzed and the results of these analyses, when used in combination with available properties for the TBC, strongly suggest that the second mechanism (b) predominates in all reasonable scenarios.
Abstract: Thermal barrier coating (TBC) systems are susceptible to delamination failures in the presence of a large thermal gradient. These failures, which occur within the TBC layer, are very different in character from those associated with the thermally grown oxide. Three possible causes of internal delamination are analyzed. In all cases, the thermomechanical properties of the TBC are allowed to vary because of sintering. (a) One mechanism relates to exfoliation of an internal separation in the TBC due to a through thickness heat flux. (b) Another is concerned with edge-related delamination within a thermal gradient. (c) The third is a consequence of sintering-induced stresses. The results of these analyses, when used in combination with available properties for the TBC, strongly suggest that the second mechanism (b) predominates in all reasonable scenarios. Consequences for the avoidance of this failure mode are discussed.

Journal ArticleDOI
TL;DR: In this article, a nanostructured zirconia coating fabricated by atmospheric plasma spraying (APS) is described, and the microstructure and phase composition of the coating was characterized with SEM, TEM, XRD and Raman Spectroscopy.
Abstract: In this paper, a nanostructured zirconia coating fabricated by atmospheric plasma spraying (APS) is described. The microstructure and phase composition of the coating was characterized with SEM, TEM, XRD and Raman Spectroscopy. In addition, the bonding strength between the nanostructured Zirconia coating and stainless steel substrate has been measured. It is found that the as-sprayed zirconia coating exhibited a bimodal distribution with small grains (60–80 nm) and large grains (70–120 nm), the later is the main microstructure of the coating. The coating is composed of tetragonal zirconia; it was found that the monoclinic zirconia existing in the starting powders transformed into tetragonal phase during plasma spraying.

Journal ArticleDOI
TL;DR: In this paper, the model proposed by Guglielmi, which does not take into account agitation of the bath, has been used and modified to explain results for Ni-PTFE deposition incorporating a correction based on the speed of magnetic rotation.
Abstract: Co-deposition of inert particles in a metallic matrix enables the production of a large range of composite materials with unique properties. Previous work on examination of the effects of the variation of experimental conditions has produced some understanding of the mechanism. When stirring is carried out using a rotating disk electrode, the hydrodynamic conditions are well defined and can be taken into account. In this paper the model proposed by Guglielmi, which does not take into account agitation of the bath, has been used and modified to explain results for Ni–PTFE deposition incorporating a correction based on the speed of magnetic rotation.

Journal ArticleDOI
TL;DR: In this paper, the effects of reactive gas pressure and flow ratio on the characteristics of the diamond-like carbon (DLC) films are systematically examined to correlate to the blood compatibility.
Abstract: Diamond-like carbon (DLC) is an attractive biomedical material due to its high inertness and excellent mechanical properties. Using plasma immersion ion implantation-deposition (PIII-D), DLC films are fabricated on silicon substrates at room temperature. By changing the C2H2 to Ar (FC2H2/FAr) flow ratio during deposition, the effects of the reactive gas pressure and flow ratio on the characteristics of the DLC films are systematically examined to correlate to the blood compatibility. The thickness, surface morphology, composition, structure, sp3/sp2 content, as well as carbon–hydrogen bonding are studied using alpha-step profilometry, atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR), respectively. The blood compatibility of the film is evaluated using in vitro platelet adhesion investigation, and the quantity and morphology of the adherent platelets are investigated employing optical microscopy and scanning electron microscopy (SEM). The Raman D-band to G-band intensity ratio is consistent with the adherent platelet quantity. Both first increase and then decrease with higher FC2H2/FAr flow ratios. This implies that the blood compatibility of the DLC films is influenced by the ratio of sp3 to sp2, not by the absolute sp3 or sp2 content. Our study suggests that DLC films with the proper sp3 to sp2 ratio and good blood compatibility can be fabricated by C2H2–Ar PIII-D using a suitable C2H2/Ar gas ratio.

Journal ArticleDOI
TL;DR: In this article, a study concerning the hardness and the Young's modulus results determined by Vickers indentation on different materials was performed, and two methods were proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP).
Abstract: Depth-sensing indentation equipment is widely used for evaluation of the hardness and Young's modulus of materials. The depth resolution of this technique allows the use of ultra-low loads. However, aspects related to the determination of the contact area under indentation should be cautiously considered when using this equipment. These are related to the geometrical imperfections of the tip, the diamond pyramidal punch and the formation of pileup or the presence of sink-in, which alter the shape and size of the indent. These and other aspects, such as the thermal drift of the equipment and the scattering at the zero indentation depth position related to surface finishing, are discussed in this work. A study concerning the hardness and the Young's modulus results determined by Vickers indentation on different materials was performed. Samples of fused silica, BK7 glass, aluminium, copper and mild steel (for which the values of Young's modulus were previously known) were tested using indentation loads in the range 10–1000 mN. Moreover, two methods are proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP). The present methods are based on: (i) analysis of the punch profile using atomic force microscopy (AFM); and (ii) a linear penetration-depth function correction (LM), based on knowledge of the values of the Young's modulus of several materials. By applying these methods to the indentation load/indentation depth results, it was possible to draw some conclusions about the benefit of the AFM and LM methods now under proposal.

Journal ArticleDOI
TL;DR: In this paper, the local dependence of stresses in the vicinity of this rough interface was investigated using a finite element (FE) method, which revealed how the parameters of roughness and the oxide thickness correlate with the stress levels.
Abstract: During thermal cyclic loading, plasma-sprayed thermal barrier coatings (TBCs) often show failure within the top coat close to the interface. In all cases this results from crack propagation of pre-existing cracks near the bond coat (BC)–top coat interface. Stresses developing on a microscopic scale near the BC–TBC interface of plasma-sprayed thermal barrier coatings govern crack growth in an initial phase of the failure process. Using a finite element (FE) method the local dependence of stresses in the vicinity of this rough interface was investigated. Measurements of real roughness profiles provided geometrical parameters needed for the calculations. A significant difference in the stress distributions was found for peak and valley locations of the BC roughness profile. The effect of BC oxidation on stress development was more pronounced in the case of less roughness. Analytical fits of the FE results revealed how the parameters of roughness and the oxide thickness correlate with the stress levels. In the next stage of research these fits will serve as input data for a microstructural based lifetime model.

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TL;DR: In this paper, high and low pressure plasma spray in a controlled atmosphere were selected as manufacturing techniques for the deposition of ceramic coatings and free-standing components for high temperature applications.
Abstract: Refractory metal borides are the object of special interest for aerospace applications requiring properties of chemical and mechanical resistance in ultra high temperature, such as nose and leading edges of re-entry space vehicles. The main objective of the research is the fabrication and characterization of plasma sprayed zirconium diboride–silicon carbide composite coatings and free-standing components for high temperature applications. High and low pressure plasma spray in a controlled atmosphere were selected as manufacturing techniques for the deposition of ceramic coatings. Fine ZrB2 and SiC precursors were agglomerated and preconsolidated into spherical, hollow powders for better flowability and silicon carbide thermal protection during the interaction with the plasma. Coatings and free-standing tubular specimens were fabricated and tested for high temperature behaviour. Thermogravimetric analysis, surface morphology investigation and high temperature X-ray diffraction showed that the addition of approximately 25% SiC induces a mechanism of self-protection of the ceramic material during heat treatment in oxidizing environments up to approximately 2100 K.

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TL;DR: In this paper, Nanocrystalline titania films were prepared by plasma electrolytic oxidation of a titanium alloy at 200-350 V in a Na2CO3 electrolytic solution using a pulsed power supply.
Abstract: Nanocrystalline titania films were prepared by plasma electrolytic oxidation of a titanium alloy at 200–350 V in a Na2CO3 electrolytic solution using a pulsed power supply. XRD, EDS and Field Emission (FE) SEM were employed to characterize the phase, composition, and microstructure of the films. Vickers indentation, nanoindentation and adhesion–tension test were used to evaluate the mechanical properties of the films. The phase, pore size and thickness of the films strongly depend on the applied voltage consistent with the previous reports. The films prepared at 350 V were porous with ∼1 μm pores and the pore walls were composed of 10–20 nm rutile crystallites. The hardness, Young's modulus and bond strength of the film were 0.9±0.2 GPa, 32±4 GPa and 37±3 MPa, respectively. The film exhibited a significant plasticity and ductility compared to the conventional coarse-grained titania ceramics.

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TL;DR: In this article, the structure and chemistry of zirconium-titanium base conversion layers were characterised as a function of immersion time in the aqueous conversion bath to understand the mechanism of film formation.
Abstract: Structure and chemistry of zirconium-titanium base conversion layers were characterised as a function of immersion time in the aqueous conversion bath to understand the mechanism of film formation. Characterisation was performed by glow discharge optical emission spectroscopy, scanning electron microscopy and transmission electron microscopy. Preferential nucleation of the zirconium-titanium oxide film and its growth occurred on and around intermetallic particles, resulting in reduced cathodic activity of the particles. Passivation of the cathodes in this manner constituted a limitation in the formation of a good quality conversion layer.

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TL;DR: In this article, the Vickers hardness could be optimized to values of polycrystalline TiC thin films, and at the same time, low friction coefficients against steel, similar to diamond-like amorphous carbon, could be realized.
Abstract: The objective of nanocomposite coatings combining hard and lubricant phases is the development of advanced multi-functional protective thin films showing abrasion resistance, and simultaneously, low friction. Up to now, no clear relation between constitution, microstructural properties and performance of such nanocomposite coatings based on dry lubricants like carbon or MoS2 has been evaluated. Deposition techniques, constitution, properties and performance of magnetron-sputtered nanocomposite coatings in the TiCC system are presented. The Vickers hardness could be optimized to values of polycrystalline TiC thin films, and at the same time, low friction coefficients against steel, similar to diamond-like amorphous carbon, could be realized. The mechanical properties and the tribological behavior of these thin films are related to the chemical composition and the microstructure of these advanced materials, characterized by electron microprobe analysis, Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy.